In an electric discharge machine, a series of voltage pulses are applied across a space, called "a machining gap," formed between the workpiece being machined and a tool electrode to intermittently generate an electric discharge thereacross. The use of a bipolar type switching transistor, or a MOS-FET type switching transistor, for pulsing the output voltage of a d.c. power supply at controlled high frequency is known. If the switching transistor fails during machining, its faulty operation may result in the flow of an overcurrent through a machining gap, which in turn may damage the workpiece being machined or cause the breakage of the wire electrode. Overheating and any conductive dust which happens to attach to the transistor circuit are possible causes such faulty operation.
Japanese Laid-Open Patent 61-111821 discloses an electric discharge machine in which a power supply is cut off when overcurrent flowing through the switching transistor is detected by a fuse or a coil.